A GENERAL PHASING ALGORITHM FOR MULTIPLE MAD AND MIR DATA
Jordi Bella and Michael G. Rossmann
Department of Biological
Sciences
Purdue University, West
Lafayette, IN 47907, USA
A general algorithm has been
developed for combining phasing information from
Multiple-wavelength Anomalous Dispersion (MAD) data collected on
multiple X-ray absorption edges, corresponding to different types
of anomalous scatterers on a given crystal, or multiple MAD data
from different derivative crystals. The algorithm also is
appropriate for combining MAD data with multiple isomorphous
replacement (MIR) data from different derivative crystals. This
algorithm is based in an alternative geometrical construction of
the Harker diagram, originally developed by Rossmann in 1961 for
the phase determination of the crystal structure of horse
oxy-hemoglobin (1). A heavy-atom phasing and refinement program
originally written by Rossmann in 1967 has been adapted to
determine protein phases by using all MAD and MIR derivatives
simultaneously. In this technique the "native" data is
considered equivalent to a "null-derivative" that
contains neither an anomalous nor an isomorphous component.
Several numerical tests will be presented for different
experimental scenarios: a double MAD experiment on (diffraction
data at two absorption edges), combination of two MAD experiments
from different derivatives, and combination of MAD data with MIR
data from multiple crystals. There is a direct relationship
between this method and the Karle equations (2) used in MAD
phasing, and it will be shown how these can be reformulated as a
particular case of the algorithm.
Cullis, A.F., Muirhead, H., Perutz, M.F., Rossmann, M.G., North, A.C.T., Proc. Roy. Soc. A265, 15-38 (1961).
Karle J. Int. J. Quantum Chem.: Quantum Biol. Symp. 7, 357-367 (1980).